The present invention relates generally to the field radio frequency (RF) power amplifiers, focusing on the aspects involved in the field when RF power amplifiers are utilized to generate high power outputs. More particularly the invention pertains to the narrow field of high power RF amplifiers having switching devices and configured similarly to those operating in a Class E mode.
Radio frequency circuit design, unlike circuit design at the lower frequencies, is a field which involves an interplay between the theoretical and the practical. While it is characterized by the same fundamental theoretical relationships well known to almost any circuit designer, across the range of frequencies involved, practical effects also become important. As a result, circuit designers in this field are often required to simultaneously understand and apply the theory of operation of each device, the function of each device as it actually operates, and the ability to experimentally attempt and reconcile results achieved. In actually fashioning circuits and devices to achieve extreme levels of performance, this latter aspect--the ability to experimentally attempt and reconcile results achieved--becomes very significant as achievements seemingly straightforward from a theoretical basis become increasingly difficult to realize. In this regard, one of the challenges faced is that of understanding and utilizing the theoretical preconceptions while remaining open minded enough to go beyond the limits described by them.
The relevant field is also unique in that seemingly insignificant changes in existing circuitry can entirely change the operation of the device. As a result of this fundamental aspect, those skilled in the art have developed a shorthand technique by which several combinations of operating parameters and resultant conditions may be characterized and utilized. This shorthand technique is that of describing amplifiers in terms of classes of operation. To the RF circuit designer, in many cases, amplification circuits which are almost identically arranged and yet function very differently can be discerned by the simple characterization of the amplifier's "class" of operation. In application, these "classes" (referred to as Class A, Class B, etc.) have become an important tool to the RF amplifier designer. For example, by merely stating that an amplifier operates in the Class E mode, those skilled in the art are able to apply such a amplifier often without the need to fully calculate theoretically the effects of that amplifier in advance of its actual application. This design technique has obviously advantages. It does, however create limitations. As it applies to the present invention, one of these limitations is that it fosters the acceptance of some preconceptions and assumed restrictions which are now proven to be unnecessary and even erroneous.
In creating RF power amplifiers, some of the goals those skilled in the art have long sought are those of higher power (powers above a few hundred watts for a single stage device), higher efficiency, and greater simplicity to lessen the cost, components, and space required for the amplifier. Each of these challenges have been addressed by those skilled in the art to varying degrees. With respect to some of these a significant advance--and a new class of operation--was invented in 1975 through U.S. Pat. No. 3,919,656. This invention, now referred to as a "Class E" amplifier, typified invention in this field. Although that amplifier was configured almost identically to its prior art, through a new selection of parameters, its operation acted differently to achieve significantly improved results. This is also true of the present invention.
As an incident of the new class of operation, however, the teachings of the Class E amplifier also resulted in a new set of preconceptions and assumed restrictions. While many of these preconceptions and restrictions made sense at the time of the original Class E invention, they continued even after their original reason for being ceased. These included an almost dogmatically pursued desire for efficiency, a theoretical design model that ignored significant internal component characteristics, and an assumed restriction on the maximum amount of power possible. In overcoming each of these facets, the present invention achieves perhaps dramatic performance advantages. With respect to the aspect of maximum power, the present invention affords a dramatic improvement. Where the prior art devices were capable of claiming the ability to consistently produce power in the range of two hundred watts and in isolated instances produce 1000 watts, without yet having identified its upper limits, the present invention can easily produce many times this amount. Where the prior art devices required the use of expensive RF switching devices in order to achieve their levels of performance, the present invention requires only inexpensive switches to achieve similar performance levels. Where the prior art devices rigidly adhered to achieving efficiency through the constraint of pursuing zero voltage at the instant of switch turn-on, the present invention departs sharply to teach a substantial voltage at such time. Perhaps most importantly, however, the present invention discloses a more accurate method of designing high power RF amplifier circuitry whereby a broad variety of improvements in performance can be achieved and whereby each of these improvements may be optimized for different applications.
Certainly other developments have attempted to improve the original Class E amplifier. In 1984, U.S. Pat. No. 4,449,174 recognized in a somewhat different field that the internal capacitance of the switch was significant for some circuits. In contrast to the present invention, however, it applied this aspect in a manner which reinforced the preconception that efficiency is paramount and can only be maintained through the zero voltage constraint at the instant of switch turn-on. That those skilled in the relevant art continued in this preconception is clear. In the 1987 RF Design article by one of the well-respected original Class E inventors entitled "Power Transistor Output Port Model," those skilled in the art were told not to stray from the zero voltage condition. In 1988 U.S. Pat. No. 4,743,858, in proposing the use of a diode for better device utilization for the Class E amplifier, again reiterated the desire to avoid voltage at the instant of switch turn-on.
The seemingly simple recognition that internal aspects of the switch were important did not alone lead to the present invention. After the 1984 patents lead, the 1985 Motorola RF Device Data article entitled "Applying Power MOSFETs in Class D/E RF Power Amplifier Design" by Helge Granburg not only presented the power level limits thought to be practically achievable (power level limits many times less than those possible through the present invention), but it also lead those skilled in the art to ignore important effects of lead inductance which were to have significant impacts on the design of the present invention. In addition, in the 1990 U.S. Pat. No. 4,891,746 in a field similar to the 1984 U.S. Pat. No. 4,449,174, the continued adherence to efficiency even taught those skilled in the art away from the important aspect of a voltage step. Although arguably in a different field, this art may have lead those skilled in the present field away from the aspect of maintaining a positive voltage across the switch which also becomes paramount at the higher power levels.
As can be seen, each of these prior efforts actually taught away from directions taken by the present invention and served to reinforce attitudes which in essence limited prior designs. This was true even though there was a well-known and long-felt need to achieve the performance of the present invention were well known and had existed. While those skilled in the art appreciated the goals of the present invention, their attempts to achieve such goals were inadequate because they failed to recognize the effect of their preconceptions. Even the unexpectedly simple and seemingly minor modifications presented by the present invention appears to have lead those skilled in the art to assume the direction taken by it would not yield the sometimes dramatic improvements achieved by it. This may also have been reinforced by the prior feeling that since an incremental increase in performance was difficult, a significant leap in performance was also difficult. By breaking from traditions long adhered to in the relevant art, the present invention proves this completely incorrect.